Catalysts containing nickel are used as the methanation catalysts, in which the active substance containing the nickel is bonded to carrier materials, for example of aluminum oxide, silicon oxide, zirconium oxide and similar. The primary, exothermic methanation reactions that take place the catalyst bed are as follows:CO+3H2⇄CH4+H2OCO2+4H2⇄CH4+2H2O
The formation of methane is accompanied by the generation of considerable heat, so that the temperatures of the reagents and the products rise as they pass through the catalyst bed. At the same time, as the temperatures rise, the equilibrium concentration of methane falls. The higher the temperature of the gas stream leaving the catalyst bed, the lower the methane content in the gas stream, correspondingly with the reaction equilibrium underlying the reaction equations given above. For this reason, in methanation reactions the reaction temperature should be kept as low as possible. The definition of a suitable temperature for the synthesis gas as it enters the catalyst bed is determined by other criteria, however. It must be borne in mind that as the temperature is lowered, the reaction rate becomes slower. In particular, it must also be noted that an inlet temperature lower than 290° C. can lead to irreversible damage to the nickel catalyst, caused as far as is known according to current understanding by a reaction between nickel and carbon monoxide to form nickel carbonyl.
In a method known from DE 29 14 806 [GB 2,018,818], which serves as the starting point for this invention, a CO conversion stage is connected upstream from the catalyst bed, which consists of a methanation catalyst, the purpose of the conversion stage being to reduce the CO content in the feed gas and thus lower the CO partial pressure to such a point that the carbonyl so damaging to the catalyst cannot form. In the conversion stage, a catalytic shift reaction takes place according to the following reaction equationCO+H2O⇄CO2+H2 
The catalyst for the CO conversion stage, which is also called a shift catalyst, contains for example two of the metals Cu, Zn and Cr, which themselves are bonded on a carrier. In the known method, both the entire synthesis gas stream and the recirculated gas stream that is split off from the product gas for cooling purposes are passed through the CO conversion stage. The shift catalyst takes up as much as 75% of the entire volume of the catalyst that is required for the methanation reactions and CO conversion, and accordingly requires significant effort in terms of technical equipment. Since the shift reaction is also exothermic, the synthesis gas is heated up during the CO conversion, so the temperature of the synthesis gas stream at the inlet to the methanation reactor rises, which also raises the outlet temperature, with the result that a low temperature at the inlet temperature into the catalytic CO shift bed is can only be used to a certain degree.